On my '70 'T', I decided to upgrade the heat exchangers w/ stainless SSIs but found they are no longer available, so I had the existing steel set blasted and painted with correct heat paint, mated to a new steel muffler painted factory grey.
Since then am enduring horrific fumes which result in dizziness, headaches and even sore throat. My tech doesn't even notice the acrid smell.
How long before the new parts calm down and the car is fume-free?
Lesson---let sleeping dogs lie.